In a typical cellular system, also referred to as a wireless communications network, wireless terminals, also known as mobile stations and/or User Equipment units (UEs) communicate via Radio Access Networks (RAN) to a Core Network (CN). The wireless terminals may be mobile stations or user equipments such as mobile telephones also known as cellular telephones, and laptops with wireless capability, e.g., mobile termination, and thus may be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node, such as a base station, which in some radio access networks is also called eNodeB (eNB), NodeB, or base station. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations. There are different types of radio network nodes/base stations, such as for example macro node/base station, pico node/base station, home eNodeB or femto base station. Typically, the types of base stations are associated with different power classes, e.g. a typical maximum transmit power of macro base station (aka wide-area base station) is above 40 dBm, whilst lower-power base stations such as pico of femto typically have the output power below 30 dBm.
The interest in deploying low-power nodes, such as pico base stations, home eNodeBs (HeNB, HBS), relays, remote radio heads, etc., for enhancing macro network performance in terms of the network coverage, capacity, and service experience of individual users has been constantly increasing over the last few years. At the same time, it has been realized the need for enhanced interference management techniques to address the interference issues caused, for example, by a significant transmit power variation among different cells and cell association techniques developed earlier for more uniform networks.
In 3rd Generation Partnership Project (3GPP), heterogeneous network deployments have been defined as deployments where low-power nodes of different transmit powers are placed throughout a macro-cell layout, implying also non-uniform traffic distribution. Such deployments are, for example, effective for capacity extension in certain areas, so-called traffic hotspots, i.e., small geographical areas with higher user density and/or higher traffic intensity where installation of pico nodes can be considered to enhance performance. Heterogeneous deployments may also be viewed as a way of densifying networks to adopt for the traffic needs and the environment. However, heterogeneous deployments bring also challenges for which the network has to be prepared for to ensure efficient network operation and superior user experience.
In heterogeneous networks, a mixture of cells of differently sized and overlapping coverage areas are deployed. A cell is a geographical area where radio coverage is provided by a base station. More than one cell can be associated with one base station. One example of such cell deployment may be a network comprising pico cells deployed within the coverage area of a macro cell. The pico cells and macro cell may each comprise a base station. A base station may be e.g. a pico base station, a macro base station, Home Base Station (HBS), radio base station, evolved node B (eNB), base station, relay, remote radio heads etc.
A base station comprises at least one antenna port, e.g. antenna port 0. Each antenna port is configured to transmit and receive signals from the base station to e.g. one or more user equipment.
Other examples of low-power nodes in heterogeneous networks are home base stations (HBS) and relays. As discussed below, the large difference in transmitted output power, e.g., 46 dBm in macro cells and less than 30 dBm in pico cells, results in an interference situation different from that seen in networks where all base stations have the same output power.
A Long Term Evolution (LTE) system uses Orthogonal Frequency Division Multiplex (OFDM) as an OFDM Access technique (OFDMA) in the downlink from system nodes to user equipments (UEs) 505, and Discrete Fourier Transform (DFT)-spread OFDM in the uplink from a user equipment 505 to an eNB. LTE channels are described in 3GPP Technical Specification (TS) 36.211 V9.1.0, Physical Channels and Modulation is described in Release 9 of LTE, among other specifications. An LTE system is used as an example in this document. However other network standards, such as GPRS, WiMAX, UMTS etc. are also applicable.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds (ms) duration, each radio frame 101 comprises ten equally-sized subframes 103 of 1 ms duration as illustrated in FIG. 1. A subframe 103 is divided into two slots, each of 0.5 ms duration. Time domain is a term used to describe the analysis of physical signals, with respect to time.
The resource allocation in LTE is described in terms of resource blocks, where a resource block corresponds to one slot in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two consecutive, i.e. in time, resource blocks represent a resource block pair and correspond to the time interval upon which scheduling operates.